1. Field of the Invention
The invention relates to semiconductor wafer processing chambers and, more specifically, to a plasma reaction chamber having a helicon wave high density plasma source.
2. Description of the Background Art
Inductively coupled RF plasma reactors are employed to perform a variety of processes on workpieces such as semiconductor wafers. One type of inductively coupled RF plasma reactor has a process chamber including a ceiling (also referred to as a top, lid or roof) and a cylindrical side wall. A pedestal supports the workpiece, such as a semiconductor wafer, so that the wafer generally lies in a workpiece support plane, and a bias RF power generator is coupled to the pedestal. A generally planar coil antenna overlies the ceiling and is coupled to a plasma source RF power generator. With a process gas in the chamber, RF power is applied to the antenna to inductively couple energy to the process gas and produce a plasma in the chamber. A chief advantage of inductively coupled RF plasma reactors over other types of reactors, such as capacitively coupled reactors, is that a higher ion density can be achieved with the inductively coupled source.
To achieve effective plasma density at the workpiece, the distance from the workpiece to the antenna must be critically selected and is generally very small, e.g., less than 7.5 cm. As the distance is decreased in order to increase the plasma density near the workpiece surface, it is known in the art that the plasma ion density decreases at the workpiece center and ultimately, at very short workpiece to antenna distances, a center null is formed in the plasma that results in an unacceptable process non-uniformity. For example, in a plasma etch process accomplished in such a reactor, the etch rate at the center of a wafer may be so much less than elsewhere that it becomes impossible to perform a complete etch across the entire wafer surface without overetching near the wafer periphery. Conversely, it becomes impossible to avoid overetching at the wafer periphery without under etching the wafer center. Thus, the problem is to find a way of increasing the plasma ion density at the workpiece surface without decreasing the antenna to workpiece distance.
One solution is disclosed in U.S. Pat. No. 4,990,229 issued Feb. 5, 1991 and incorporated herein by reference. This patent discloses a high density plasma generator that utilizes a helical antenna that produces helicon waves to excite a plasma. The plasma is formed in a relatively small bell jar having an open end attached to an opening in the ceiling of a process chamber. The chamber is circumscribed with a plurality of permanent magnets that form a plasma confining magnetic field referred to as a "magnetic bucket". The chamber is typically a cylindrical enclosure having a top with the bell jar mounted centrally therein and a cylindrical side wall. A pedestal is mounted at the bottom of the chamber to support a workpiece, such as a semiconductor wafer, beneath the opening of the bell jar.
The bell jar, a non-conducting cylindrical chamber that is fabricated of quartz or Pyrex, is encircled by an antenna that is coaxially aligned with a center axis of the bell jar. The antenna is a pair of spaced apart loops that are electrically connected to one another such that RF current flows in an opposite direction through each loop. This combination of a bell jar and antenna forms a high density plasma source. The loop antenna is coupled to a 13.56 MHz RF source and, when energized, produces helicon waves having a "mode zero" (m=0) mode structure for the magnetic and electric fields within the bell jar. A process gas is supplied to the bell jar and infused with energy from the helicon waves to form a high density plasma.
Plasma control within the bell jar is provided by a plurality of electromagnets that circumscribe the exterior of the bell jar. The magnets form an axial magnetic field that transfers the plasma from the plasma source to the magnetic bucket.
To contain the plasma as it exits the bell jar and enters the bucket, the magnetic bucket is surrounded with strips of vertically oriented permanent magnets that extend along the cylindrical side walls of the bucket from the top to the bottom of the bucket. Each magnet is oppositely polarized from each adjacent magnet such that a magnetic field extending from one magnet to an adjacent magnet penetrates into the interior of the bucket by approximately a centimeter to confine the plasma in the bucket to a central region extending from the source to the workpiece.
To accelerate the plasma from the bell jar toward the workpiece, the pedestal is biased with an RF signal relative to ground. The chamber, including the side walls, top, and bottom are grounded. As such, the pedestal forms a cathode electrode and the chamber forms an anode electrode.
During, for example, plasma etching within this form of high density plasma reactor, the byproducts of the etch processes can become deposited upon many of the surfaces within the chamber including the side walls, top and bottom. Such deposition can change the electrical structure of the anode electrode such that the RF bias characteristics are altered. Such an alteration in the bias characteristics can change the plasma uniformity during wafer processing causing an unpredictable plasma fluctuation that can ruin wafers and cause anomalous etching. Additionally, such deposits on the walls and top of the chamber can create contaminants that flake and fall upon the wafer when the temperature of the process chamber changes during and after processing. Furthermore, such uncontrolled deposition can result in difficult cleaning that may lead to a build up of contaminants in the chamber over the life of the chamber resulting in shortened useful life of the chamber.
The bell jar and chamber are not generally cooled to any substantial degree. Usually a fan is provided to blow air over the bell jar. Such a fan is not sufficient to maintain the bell jar and chamber at a constant temperature during processing of a wafer and after processing a wafer. If the temperature of the chamber fluctuates by more than 10 degrees C., the material deposited on the walls of the chamber will flake and dislodge from the walls. Such particulates can contaminate the present wafer or those wafers that are subsequently processed in the chamber. Furthermore, temperature fluctuations of more than 10 degrees C. cause proportional fluctuations in process rates. Such process rate fluctuations are disadvantageous to wafer processes that are used to form semiconductor devices having line widths of less than 0.35 microns.
Therefore, there is a need in the art for a high density RF plasma reactor having improved plasma and thermal control that leads to improved wafer processing and contaminant performance.